The present invention relates to turbine buckets having airfoil tip shrouds and particularly relates to a shroud having a tip seal and a cutter tooth located intermediate opposite ends of the shroud in the direction of rotation of the bucket.
Airfoils on turbine buckets are frequently provided with tip shrouds. The shroud prevents failure of the airfoil in high cycle fatigue due to vibratory stresses. Seals are typically provided which project radially outwardly from the outer surfaces of the shrouds and extend between opposite ends of the shrouds in the direction of rotation of the turbine rotor. The tip shroud seals conventionally extend into a groove formed in a stationary shroud opposing the rotating tip shroud. The stationary shroud has a honeycomb pathway. Rather than providing a zero tolerance seal between the tip shroud and the stationary shroud, resulting in instability of the airfoil, it has been found desirable to provide a leakage path over the tip shroud seal which will remove such instability. Typically, a cutter tooth is provided at the leading edge of the tip shroud to cut a wider groove in the honeycomb of the stationary shroud than the width of the tip seal. This enables leakage flow between the high and low pressure regions on opposite sides of the seal extending in the groove. While this disadvantageously results in a decrease in pressure drop across the airfoil with resulting diminishment of sealing capability, the sacrifice in efficiency is compensated by the increase in stability of the airfoil.
However, because the mass of the tooth is not located in the same radial line as the center of mass of the airfoil, it has been discovered that a high stress is induced along the fillet region between the airfoil and the tip shroud. This increased stress at high temperatures leads to a high creep rate on the shroud and ultimately can result in failure of the shroud, for example, by cracking or splitting. It will be appreciated that the failure of a single bucket shroud causes the turbine necessarily to be taken off-line. Consequently, shroud failure due to increased stress at the fillet region between the tip shroud and the airfoil requires time-consuming and costly repairs, including bringing the turbine off-line, i.e., downtime, in addition to the labor and replacement parts necessary to effect the repair.
In accordance with a preferred embodiment of the present invention, the cutter tooth on the airfoil tip shroud is located intermediate the opposite ends of the shroud and preferably in substantial radial alignment with the center of mass of the airfoil. The cutter tooth is thus located substantially medially of the length of the shroud in the circumferential direction and closely adjacent to a radial line through the center of mass of the blade. Consequently, the cutter tooth in the center of the shroud enables the mass of the cutter tooth to be more in coincidence with the radial line intersecting the center of mass of the airfoil. This, in turn, minimizes the moment generated by the additional mass of the cutter tooth, resulting in lower fillet stress. The reduced stress extends creep life of the fillet which is frequently the life-limiting location of the part.
In a preferred embodiment according to the present invention, there is provided a turbine bucket comprising an airfoil having a tip shroud, a seal projecting radially outwardly from the shroud and extending continuously between end edges of the shroud in a direction of rotation of the airfoil about a turbine axis, a cutter tooth carried by the shroud and projecting to at least one side of the seal in a direction generally normal to the turbine axis for cutting a groove in an opposing fixed shroud, the cutter tooth having a discrete length in the direction of the seal shorter than the length of the seal and located intermediate the ends of the shroud.
In a further preferred embodiment according to the present invention, there is provided a turbine bucket comprising an airfoil having a tip shroud, a seal projecting radially outwardly from the shroud and extending continuously between end edges of the shroud in a direction of rotation of the airfoil about a turbine axis, a cutter tooth carried by the shroud and projecting to at least one side of the seal in a direction generally normal to the turbine axis for cutting a groove in an opposing fixed shroud, the cutter tooth having a discrete length in the direction of the seal shorter than the length of the seal and located substantially medial of the length of the seal.
In a further preferred embodiment according to the present invention, there is provided a turbine bucket comprising an airfoil having a tip shroud, a seal projecting radially outwardly from the shroud and extending continuously between end edges of the shroud in a direction of rotation of the airfoil about a turbine axis, a cutter tooth carried by the shroud and projecting to at least one side of the seal in a direction generally normal to the turbine axis for cutting a groove in an opposing fixed shroud, the cutter tooth having a discrete length in the direction of the seal shorter than the length of the seal and lying substantially in radial alignment with a radial line through the center of mass of the airfoil.